[docs] Added missing image
[docs] Added missing image

Add the data flow diagram to the list of images to be packaged.

Pecisk/libgdata-gtasks,pwithnall/libgdata,Pecisk/libgdata,Pecisk/libgdata,GNOME/libgdata,pwithnall/libgdata,Distrotech/libgdata,Distrotech/libgdata,Pecisk/libgdata,Pecisk/libgdata-gtasks,Distrotech/libgdata,Pecisk/libgdata-gtasks